Qualifying telephone lines for data transmission

ABSTRACT

The method assesses the suitability of customer telephone lines ( 12, 13, 14 ) for data transmission. The method includes selecting a telephone line having tip and ring wires by means of a computer ( 30 ) and switch ( 15 ), and electrically connecting the tip and ring wires together at a test access ( 29 ) adjacent one end of the selected line to produce a common mode configuration. Single-ended electrical measurements are performed on the wires in the common mode configuration by a measurement unit ( 27 ) connected to the test access ( 29 ) to determine an electrical property of the wires from the measurements.

BACKGROUND OF THE INVENTION

This invention relates generally to telephone lines, and moreparticularly, to qualifying telephone lines for data communications.

Public switched telephone networks, e.g., plain old telephone systems(POTS), were originally designed for voice communications having alimited frequency range. Today, the same POTS networks often carry datatransmissions using higher frequencies. The difference in frequenciessuggests that some POTS lines, which function well for voice, willfunction poorly for data. The risk of poor quality data transmissionshas motivated telephone operating companies (TELCO's) to develop testsfor predicting the quality of lines for data transmissions.

One quality test uses physical line length to determine a line'sattenuation. The attenuation of a line whose length is less than aboutfour kilometers (km) is usually low enough for data transmission. But,measuring the line length is typically more involved than measuring thestraight line distance between a customer's address and a switchingstation. Typically, lines form branching structures rather than goingradially from the switching station to the customer addresses. Thus,determining a line length usually entails manually mapping the actualbranching structures connecting the customer to the switching station.Such complex manual techniques can be time intensive and may lead toerrors.

Furthermore, determining that a line's length is less than a preselectedlimit, e.g., four km, may be insufficient to qualify the line for datatransmission. The line's attenuation also depends on the physicalproperties of each branch segment making up the line, e.g., the gaugemixture of the line. In lines having segments with different properties,the above-described mapping technique generally should take into accountthe properties of each segment to determine the total attenuation of theline.

TELCO's have also used direct electrical tests to determine the qualityof POTS lines for data transmissions. Typically, such tests are manualand two-ended. Two-ended tests involve sending one employee to acustomer's address or final distribution point and another employee to aswitching station. The two employees coordinate their activities toperform direct electrical measurements on the customer line usinghand-held devices. These two ended measurements are substantiallyindependent of the termination characteristics at the customer'saddress. An example of two-ended measurements is described inROEHRKASTEN W: ‘MESSUNG VON XDSL-PARAMETERN’ NACHRICHTENTECNIKELEKTRONIK, DE, VEB VERLAG TECHNIK. BERLIN, vol. 48, no. 2, 1 Mar. 1998(1998-Mar.-01), pages 20–21, XP000752845 ISSN: 0323-4657.

Nevertheless, these two-ended tests need two separate employees, whichmakes them labour intensive. The labour requirements affect the cost ofsuch tests. The two-ended tests cost about $150 per customer line. Thiscost is so high that a TELCO is often prohibited from using such testsfor all customer lines.

HEDLUND, ERIC; CULLINAN, TOM: ‘DSL Loop Test’ TELEPHONY, vol. 235, no.8, 24 Aug. 1998 (1998-Aug.-24), pages 48–52, XP002147002 USA, mentionssingle-ended testing but does not specify how such testing may beperformed.

The present invention is directed to overcoming, or at least reducing,one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a method of assessing thesuitability of customer telephone lines for data transmission. Themethod includes selecting a telephone line via a test access of aswitching station and electrically connecting the tip and ring wiresadjacent one end of the selected line in a common mode configuration.The method also includes performing single-ended electrical measurementson the tip and ring wires with respect to ground by driving the tip andring wires in the common mode.

The method includes determining an electrical property of the wires fromthe single-ended measurements.

In a second aspect, the invention provides a system for determining asignal attenuation of a customer line. Each customer line has tip andring wires. The system includes a measurement unit having first andsecond input terminals to couple to a test access of a telephony switch.The measurement unit is capable of driving the input terminals in acommon mode configuration with respect to ground to perform single-endedimpedance measurements on the tip and ring wires of the customer linesin the common mode configuration.

In a third aspect, the invention provides a method of marketing customertelephone lines for selected data transmission services. Each line hasassociated tip and ring wires. The method includes automaticallyperforming single-ended electrical measurements on the customertelephone lines and determining which of the customer lines qualify fora selected data transmission service from the measurements. The tip andring wires are driven in a common mode configuration during at least aportion of the measurements upon the associated lines. The methodincludes sorting the lines by distribution point and qualification totransmit data. The method also includes offering the selected dataservice to a portion of the customers in response to lines determined tobe qualified for the service being available.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the invention will beapparent from the following description, taken together with thedrawings in which:

FIG. 1 shows an embodiment of a system for testing the suitability ofcustomer lines for data transmission;

FIG. 2 shows the segments of one customer line from FIG. 1;

FIG. 3 is a flow chart illustrating a method of testing telephone linesfor data transmission;

FIG. 4 shows a portion of the measurement unit that performs impedancemeasurements on the lines of FIG. 1;

FIG. 5 is a flow chart for a method of qualifying customer lines usinglow frequency measurements on tip and ring wires driven in a common modeconfiguration with respect to ground;

FIG. 6 is a table comparing attenuations found with the methods of FIG.5 to reference values; and

FIG. 7 is a flow chart illustrating a method of marketing datatransmission services for customer lines.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a portion of a POTS network 8. The network 8 includescustomer lines 12–14 connecting customer units 16–18, i.e., telephonesand/or modems, to a telephony switch 15 located in a TELCO centraloffice 10. The switch 15 may be a POTS switch or any other device forconnecting the lines 12–14 to the telephone network 8, e.g., a digitalsubscriber loop access multiplexer (DSLAM) (not shown).

Each customer line 12–14 is a twisted copper two-wire pair adapted fortelephone voice communications. The two wires of each line 12–14 aregenerally referred to as the ring and tip wires. The lines 12–14 arecontained in one or in a series of standard telephony cables 20. Thecable 20 may carry more than a dozen customer lines (not all shown)thereby creating an environment that changes electrical and transmissionproperties of the separate lines 12–14. The properties of the lines12–14 may also depend on their segment structure.

FIG. 2 shows that the customer line 12 has several paired coppertwo-wire segments 21–23. The segments 21–23 are located in separatecables 20, 24–25 and couple serially through couplers 26. Each segment21–23 may have a different length and/or gauge than the other segments21–23. The segmented structure of the line 12 can affect electricalproperties, e.g., the signal attenuation.

Referring again to FIG. 1, single-ended measurements on the lines 12–14are performed with a measurement unit 27 located in the central office10. The measurement unit 27 couples, via a line 28, to a standard voicetest access 29 of the switch 15. The test access 29 provides electricalcouplings to selected customer lines 12–14 in a voice frequency range ofat least between 300 Hertz (Hz) and 4 kilo-Hz (KHz), i.e., a lowfrequency range. The measurement unit 27 uses the test access 29 toperform a single-ended measurements on the lines 12–14, e.g., impedancemeasurements.

The line testing is controlled by a computer 30. The computer 30 sendssignals the switch 15, via line 31, e.g., to select the line 12–14 to betested. The computer 30 sends signals to the measurement unit 27, vialine 32, to select and control the test to be performed. The measurementunit 27 sends measurement results to the computer 30 via the same line32.

The computer 30 includes a storage medium 33 encoding an executablesoftware program for testing selected ones of the lines 12–14. Theprogram includes instructions for one or more methods of controllingsingle-ended measurements on the lines 12–14. The program also includesinstructions for methods of analyzing the measurements to qualify ordisqualify the lines 12–14 for data transmissions. Both types of methodare described below.

The line testing qualifies or disqualifies the individual lines 12–14being tested. To qualify, the computer 30 must predict that the line12–14, under test, will support data transmissions without remedialmeasures. To disqualify, the computer 30 must predict that the line12–14, under test, will not support data transmissions without remedialmeasures. The computer 30 may perform tests before or after the line12–14, under test, is in service for data transmissions.

Tests to qualify or disqualify a line 12–14 for data transmissioninvolve several steps. For each step, the computer 30 signals the switch15 to disconnect the particular line 12–14, selected for testing, fromthe line card (not shown) and reroute the line to the test access 29.When the switch 15 reroutes the line, the computer 30 signals themeasurement unit 27 to perform preselected single-ended measurements onthe selected line 12–14. The measurement unit 27 performs themeasurements and returns results to the computer 30. After receiving theresults of the measurements, the computer 30 signals the switch 15 toreroute the selected line 12–14 to the line card. Then, the switch 15transfers connections for the selected line 12–14 to the line cardenabling the associated customer unit 16–18 to again communicate withthe rest of the network 8.

FIG. 3 is a flow chart illustrating a method 50 for determining thesuitability of a selected one of the customer lines 12–14 for apreselected data transmission service. By way of example, the line 12 ofFIG. 1 is selected, but any of the lines 12–14 can be evaluated by themethod 50. Each step of the method 50 includes one or more single-endedmeasurements on the selected line 12 and an analysis of the measurementsby the computer 30 as has been already described. In addition, the stepsof the method 50 fall into two stages.

In the first stage, the computer 30 tests for traditional line faults byperforming independent electrical measurements on the tip and ring wiresT, R of the selected line 12. First, the computer 30 performs suchmeasurements to determine whether the selected line 12 has any metallicfaults (step 52). Metallic faults include shorts to ground, to a voltagesource, or between the paired wires T, R, and/or capacitive imbalancesbetween the paired wires T, R of the selected line 12. Second, thecomputer 30 performs such measurements to determine whether the selectedline 12 has any speed inhibiting faults (step 54). Speed inhibitingfaults include resistive imbalances between the paired wires T, R of theselected line 12, and split pair or load inductances. Speed inhibitingfaults also include bridged taps that reflect signals resonantly, e.g.,the spurious tap 55 shown in FIG. 2, and elevated line-noise levels.

The threshold values of single-ended measurements, which indicate theabove-described faults, generally depend on the type of datatransmissions. Methods for performing and analyzing such single-endedmeasurements are known in the art. For example, U.S. Application No.60/106,845 ('845), filed Nov. 3, 1998, by Roger Faulkner et al, and U.S.Pat. Nos. 5,699,402 ('402) and 4,113,998 ('998) describe such methodsand apparatus. The '845 application and '402 and '998 patents areincorporated by reference, in their entirety, in the presentapplication. The '402 application and the '402 and '998 patents alsodescribe apparatus 53, of the measurement unit 27 used for thesingle-ended measurements to detect the faults.

If the computer 30 to finds either a metallic or a speed-inhibitingfault, the computer 30 disqualifies the selected line 12 for datatransmissions (block 56). If the computer 30 finds no such faults, thecomputer 30 determines whether the selected line 12 attenuates signalsof a selected frequency by more than a threshold value for thepreselected data transmission service (step 58). In the absence offaults, the signal attenuation at high frequencies is the primarymeasure for determining a line's ability to transmit data.

FIG. 4 shows portions of the measurement unit 27 for measuring theimpedances subsequently used to determine the attenuation of theselected customer line 12. The measurement unit 27 includes an AC signalgenerator 36, which provides an AC driving voltage and current formeasuring the impedances. During the measurements, the AC signalgenerator 36 drives two input terminals 40, 41 in a common modeconfiguration. The input terminals 40, 41 electrically connectinternally at a point 43 to produce the common mode configuration. Theterminals 40, 41 also couple, via the line 28, to the test access 29 ofthe switch 15. The measurement unit 27 also has a voltmeter 38 tomeasure the driving voltage with respect to ground, and an ammeter 40 tomeasure the driving current in the common mode configuration.

The test access 29 has internal connections 44, which electricallycouple the tip and ring wires T, R of the line 12 under test to theterminal 40 and the terminal 41, respectively. Thus, the tip and ringwires T, R are electrically connected together, at the switch end, sothat the signal generator 36 drives these wires T, R in common modeconfiguration during impedance measurements. Driving the wires T, R incommon mode makes electrical measurements insensitive to terminationcharacteristics of the customer unit 16.

Both the preselected threshold value for the signal attenuation and thepreselected frequency depend on the type of data transmission. For ISDNdata transmissions, the preselected threshold is about 45 deci-Bells(dB) at 100 KHz. For ASDL data transmissions, the preselected thresholdis about 40 dB at 300 KHz depending on deployed terminal equipment.

If the signal attenuation at the preselected frequency is abovethreshold, the computer 30 disqualifies the selected line 12 for thecorresponding type of data transmissions (block 56). If the signalattenuation is below threshold at the preselected frequency, thecomputer 30 qualifies the line 12 for the corresponding type of datatransmissions (block 60) providing no faults were found at either step52 or step 54.

FIG. 5 illustrates one method 70 of determining whether the signalattenuation for the selected line 12 is above the threshold in step 58of FIG. 4. First, the measurement unit 27 performs single-endedcommon-mode measurements of the capacitance C and the impedance Z of theselected line 12 as described with relation to FIG. 3 (step 72). Themeasurements of C and Z are typically low frequency measurements, i.e.,between about 300 Hz to 4 KHz, because the standard test access 29 ofthe switch 15 does not necessarily support high frequency measurements.If the test access 29 supports higher frequency measurements, suchfrequencies can be used to set a better resolution on the high frequencyattenuation of the selected line 12.

The measurement unit 27 measures the capacitance C and then uses thevalue of C to determine the frequency for measuring the impedance Z. Thecapacitance C is a lumped value between the common mode tip and ringwires T, R and ground. The measurement unit 27 determines C at a lowfrequency, e.g., 80 Hertz (Hz). If the measured value of C is less than400 nano-Farads (nF), the AC signal generator 27 drives the tip and ringwires T, R in common-mode at about 2.5 KHz to measure the impedance Z.If the value of C is greater than 400 nF, the AC signal generator 27drives the tip and ring wires T, R, in common-mode, at a higherfrequency between about 3 and 20 KHz, e.g., 3.0 KHz, to measure theimpedance Z. The computer 30 uses the relation Z=V/I, where the voltageV is measured by the voltmeter 38 and the current I is measured by theammeter 40, to find Z.

Next, the computer 30 determines the signal attenuation A(f) at highfrequencies characteristic of data transmissions using the low frequencymeasurements of C and Z (step 74). The high frequencies are more thanten times the frequencies used for measuring Z and C. The value of“A(f)” at higher frequency “f” is known from an empirical formula (1)given by:A(f)=K[Z ²+(2πfC)⁻²]^(−1/2).  (1)The value of K=5,000 dB-ohms provides good predictions of theattenuation A(f), in dB, for C and Z (in ohms) measured at lowfrequencies as described above. For this value of K, the frequency f, atwhich the attenuation is to be determined, should be between about 40KHz and 300 KHz.

Next, the computer 30 determines whether the high frequency attenuationA(f) is above the preselected threshold for the selected type of datatransmissions (step 76). If the attenuation A(f) is above the threshold,the computer 30 disqualifies the selected line 12. If the attenuation isbelow threshold, the computer 30 qualifies the selected line for theselected data transmissions.

FIG. 6 shows a table 80 comparing values of the signal attenuation A, indB, at high frequencies, found using the method 70, to reference values,found by an independent method, i.e., simulators. Column 3 of table 80shows the values of A(f) predicted from low frequency measurements of Cand Z and the formula (1). Column 4 of table 80 shows the values of A(f)obtained from simulators of customer lines, i.e., the reference values.The values of attenuation A(f) of FIG. 6 are given in dB's at afrequency “f” of about 100 K Hz.

The values of the high frequency attenuation A(f) of the table 80correspond to a variety of one and two segment structures for theselected customer line 12. Columns 1 and 2 list segment lengths andgauges, i.e., diameters in millimeters, for the copper tip and ringwires T, R of the selected line 12. For each one and two segmentstructure shown, the predicted and reference attenuations differ by lessthan about 2 dB. Generally, formula (1) gives values of the highfrequency attenuation A, which differ by less than about 3 dB forvarious segment mixtures if the wire gauges are between about 0.4 mm and0.7 mm and total line lengths are less than about 6.5 km.

FIG. 7 is a flow chart illustrating a method 90 of marketing preselecteddata transmission services for the customer lines 12–14 of FIG. 1.First, a TELCO performs common-mode single-ended electrical measurementson the selected group of lines 12–14 as described in relation to FIG. 3and step 70 of FIG. 5 (step 92). Next, the TELCO determines which of thelines 12–14 qualify for the preselected data service from themeasurements (step 94). This determination includes performing the steps74 and 72 of the method 70 of FIG. 5 and may include the steps 52 and 54of the method 50 of FIG. 4. The determination includes sorting the linesbased on their final distribution points and qualification status forthe preselected data transmission service. Next, the TELCO offers thepreselected data transmission service to the portion of the customers towhich the lines 12–14 qualified in step 94 are available, i.e.,customers at final distribution points with qualified lines (step 96).The TELCO connects a portion of the qualified lines 12–14 to thecustomers who subsequently request the offered data services (step 98).The TELCO also bills usage for a portion of the lines 12–14 at pricesthat depend on whether the lines 12–14 qualify or disqualify for thedata transmission services (step 100).

To provide the requested data services at step 98, the TELCO may swapcustomer lines to the same final distribution point. The swappingreassigns a qualified line to a customer requesting data service if thecustomer's own line is disqualified. The swap reassigns the customer'soriginal disqualified line to another customer, who is at the same finaldistribution point and not demanding data service. The disqualified linecan still provide voice services to the other customer. Thus, swappingcan increase a TELCO's revenue by making more lines available for moreexpensive data services.

A TELCO can also use swapping in response to a request by the customerfor data services. In response to such a request, the TELCO determineswhether the customer's own line qualifies for the requested service bythe above-described methods. If the line qualifies, the TELCO providesthe customer data services over his own line. If the line disqualifiesfor the requested service, the TELCO performs additional qualificationtests on other lines to the same final distribution point, which are notpresently used for data transmission services. If one of those linesqualifies for the requested data service, the TELCO swaps the customer'sline with the qualified line. Then, the qualified line provides dataservices to the customer requesting such services and the unqualifiedline provides normal voice service to the other customer.

Other embodiments are within the scope of the following claims.

1. A method of assessing the suitability of customer telephone lines fordata transmission, comprising: selecting a telephone line having tip andring wires via a test access of a switching station; electricallyconnecting the tip and ring wires together adjacent one end of theselected line to form a common mode configuration; performingsingle-ended electrical measurements by driving the wires in the commonmode configuration with respect to ground; and determining an electricalproperty of the wires from the single ended measurements.
 2. The methodof claim 1, wherein the determining comprises finding an impedance (Z)of the wires in the common mode configuration.
 3. The method of claim 2,wherein the performing comprises driving the wires at low frequenciesand the act of the determining finds a property at a high frequency, thehigh frequency being at least ten times the highest one of the lowfrequencies.
 4. The method of claim 2, wherein the determining comprisescalculating an attenuation from the impedance.
 5. The method of claim 4,wherein the measuring comprises finding a capacitance (C) for the tipand ring wires in the common mode configuration.
 6. The method of claim5, wherein the calculating uses a formula to obtain the attenuation(A(f)), the formula being A(f)=K [Z²+(2πfC)⁻²]^(−1/2), the f being thefrequency, and the K being a number.
 7. The method of claim 2, furthercomprising: determining whether the selected line has a line fault; anddisqualifying the line in response to finding the line fault.
 8. Themethod of claim 2, wherein the fault is a speed inhibiting fault.
 9. Themethod of claim 8, wherein the speed inhibiting fault includes one of aresistance imbalance, a bridged tap, a load coil, and a noise levelabove a preselected threshold.
 10. The method of claim 8, wherein theline fault includes a metallic fault.
 11. The method of claim 10,wherein the metallic fault includes one of a capacitance imbalance, ashort to ground, a short to a voltage source, and an intermediate shortbetween the tip and ring wires.
 12. The method of claim 10, furthercomprising: determining whether the selected line has a speed inhibitingfault; and disqualifying the line in response to finding the speedinhibiting fault.
 13. The method of claim 8, wherein the act ofdetermining an electrical property includes calculating an attenuationfor the line using the electrical measurements.
 14. A system fordetermining signal attenuations of customer telephone lines, each linehaving tip and ring wires, comprising: a measurement unit having firstand second input terminals to couple to a test access of a telephonyswitch, the measurement unit capable of driving the input terminals in acommon mode configuration to perform single-ended impedance measurementson the tip and ring wires of the customer lines.
 15. The system of claim14, wherein the measurement unit further comprises: a voltmeter coupledto measure a voltage driving said input terminals in the common modeconfiguration; and an ammeter coupled to measure a current going to saidinput terminals in the common mode configuration.
 16. The system ofclaim 15, wherein the measurement unit further comprises: a signalgenerator connected to the first and second terminals to drive saidterminals in the common mode configuration.
 17. The system of claim 15,wherein the measurement unit further comprises apparatus to performsingle-ended measurements to detect one of metallic faults and speedinhibiting faults on the customer lines.
 18. The system of claim 14,further comprising: a processor coupled to the measurement unit andcapable of coupling to the switch, the processor having a data storagemedium encoding a program of instructions for a method, the methodcomprising: ordering the measurement unit to perform the single-endedmeasurements; and analyzing results of the ordered measurements todetermine a signal attenuation of the one of the customer lines.
 19. Thesystem of claim 18, wherein the method further comprises: determiningwhether the one of the lines is qualified to transmit data from thesignal attenuation.
 20. The system of claim 18, wherein the signalattenuation corresponds to a frequency at least ten times frequencies atwhich the measurement unit is capable of driving the one of the linesthrough the test access.
 21. The system of claim 18, wherein the methodfurther comprises: ordering the switch to transfer connections for theone of the lines from the network to the test access prior to the act ofordering the measurement unit.
 22. The system of claim 14, furthercomprising: the switch having the test access, the switch being acentral office switch.
 23. The system of claim 17, wherein the testaccess is adapted to transmit electrical signals having voice-rangefrequencies.
 24. A program storage device encoding an executable programof instructions for a method of determining the signal attenuation ofcustomer telephone lines connected to a central switch, the methodcomprising: ordering the switch to transfer connections for one of thelines from the network to a test access of the switch; ordering ameasurement unit to perform single-ended impedance measurements on tipand ring wires of one of the lines by driving the tip and ring wires ina common mode configuration using the test access; and analyzing resultsof the ordered measurements to determine a signal attenuation of the oneof the customer lines.
 25. The device of claim 24, wherein the methodfurther comprises: determining whether the one of the lines is qualifiedto transmit data from the signal attenuation.
 26. The device of claim24, wherein the signal attenuation corresponds to a frequency (f) atleast ten times signal frequencies of the single-ended measurements. 27.The device of claim 26, wherein the act of analyzing comprises:calculating the attenuation (A) based on a formula, the formula beingA(f)=K [Z²+(2πfC)⁻²]^(−1/2), and wherein Z and C are the respectiveimpedance and capacitance of the line in the common mode configuration.28. The device of claim 24, the method further comprising: determiningwhether the selected line has a line fault; and disqualifying the linein response to determining that the line has a fault.